1. Field of the Invention
The present invention relates to a four-wheel drive work vehicle including a pair of right and left front wheels rotatably driven by receiving power from an engine via a differential mechanism and a pair of right and left non-steerable rear wheels rotatably driven by receiving power from the engine via right and left side clutches, the side clutches being operated by a clutch operating member which is controlled based on a steered displacement of the front wheels.
2. Description of the Related Art
With a four-wheel drive lawn mower, in general, the right and left front wheels as steerable wheels are driven via a differential mechanism whereas the rear wheels as non-steerable wheels are driven via a differential mechanism having a differential lock function (see e.g. Japanese Patent Application “Kokai” No.: 2003-106412). Further, the convention has also proposed a construction which provides side brakes for allowing independent braking of right and left rear wheels which are driven via the differential mechanism (see e.g. Japanese Patent Application “Kokai” No.: 2000-53032).
With the former type of construction which drives both the front and rear wheels via respective differential mechanisms, a speed difference is provided between the inner-side wheels and outer-side wheels according to a steered amount of the vehicle. Hence, this construction is advantageous in permitting reliable and smooth vehicle steering with the four-wheel drive. This construction, however, is disadvantageous in that the minimum turning radius of the vehicle tends to be relatively large, which presents difficulty in an operation on a limited lawn-covered land or a land having a complicated ground contour. On the other hand, with the latter type of construction having side brakes for the right and left rear wheels, a small-radius (sharp) turn is made possible by applying, when needed, a braking force to one rear wheel which is located on the inner side of that turn. Hence, this construction is advantageous in that such operation on a limited lawn-covered land or a land having a complicated ground contour can be done very easily.
However, when the vehicle effects a small turn by applying a braking force to the inner side rear wheel with the side brake, there occurs a forcible “dragging” displacement of the rear wheel locked by the side brake, which displacement tends to roughen up the traveling ground or damage the lawn in the case of a lawn mower vehicle. If the vehicle is constructed as a rice planting machine, such small turn roughens the land at the ridge end of the field where the turn is effected.
In order to solve this problem, from Japanese Patent Application “Kokai” No.: 2003-170859, there is known a further construction wherein the right and left steerable front wheels are driven via a differential mechanism whereas the non-steerable rear wheels are driven via respective side clutches and the side clutches and a steering mechanism are mechanically coupled to each other so that the side clutch for the rear wheel on the inner side of the turn is automatically disengaged for rendering this rear wheel freely rotatable in response to a steering operation of the front wheels by an angle exceeding a predetermined angle from the straight traveling condition. With this construction, since the inner rear wheel is rendered freely rotatable, there occurs no dragging displacement in the rear wheel which would otherwise occur in its locked condition and the small turn can be effected relatively easily, while not roughening the traveling ground much. This steering construction configured to automatically disengage the side clutch for the rear wheel on the inner side of the turn allows even an inexperienced or unskilled operator to effect the small turn with three wheel drive mode relatively easily by simply turning the steering wheel. This is a significant advantage in practical terms. This construction, however, has a structural difficulty as follows. Namely, with this conventional construction, when the front wheels (steerable wheels) are steered by an angle beyond the predetermined angle, a clutch lever for operating the side clutch for the inner side rear wheel is operated to a clutch disengaging position. For further steering operation subsequent to the side clutch disengagement, a “stroke absorbing” compression spring incorporated in a link mechanism for clutch operation is elastically deformed so as to prevent the clutch lever from being operated more than necessary. In this case, the stroke absorbing spring is compressed in advance so as to allow the clutch lever to be disengaged against an engaging urging force for the side clutch. Hence, the stroke absorbing operation requires further or additional compression deformation of this compression spring. For this reason, in the case of this conventional construction, after the steering wheel is operated to an operational range sufficient to disengage the side clutch, the operational load for further operating the steering wheel is progressively increased. If the vehicle has no power steering function, this provides a heavy operational feel to the operator in the steering operation.